KVM: selftests: SYNC after guest ITS setup in vgic_lpi_stress

KVM: selftests: SYNC after guest ITS setup in vgic_lpi_stress

vgic_lpi_stress sends MAPTI and MAPC commands during guest GIC setup to
map interrupt events to ITT entries and collection IDs to
redistributors, respectively.

We have no guarantee that the ITS will finish handling these mapping
commands before the selftest calls KVM_SIGNAL_MSI to inject LPIs to the
guest. If LPIs are injected before ITS mapping completes, the ITS cannot
properly pass the interrupt on to the redistributor.

Fix by adding a SYNC command to the selftests ITS library, then calling
SYNC after ITS mapping to ensure mapping completes before signal_lpi()
writes to GITS_TRANSLATER.

Signed-off-by: Maximilian Dittgen <mdittgen@amazon.de>
Link: https://msgid.link/20251119135744.68552-2-mdittgen@amazon.de
Signed-off-by: Oliver Upton <oupton@kernel.org>